There is a general need for devices preventing return flow, for instance under conditions of underpressure, in order to prevent the penetration of air into a supply line, or the return to the supply duct of a liquid which had already passed through. With regard to water supply applications, there is a large interest in preventing a return flow of water into the supply lines, since the water might then lose its drinking quality.
In order to prevent such return flows it is known to use check valves whose movable valve closing members are subjected to spring action in the direction of a valve seat which is provided in the housing of the check valve.
In order to protect the valve control elements against the incursion of for instance Teflon-hemp (packing) residues, chips or similar dirt particles, which could affect the operational safety of the valve, it has been long known to arrange dirt trapping filters in water fittings, upstream of the valve control elements, these filters being separate components to be manufactured and mounted separately, which is not only expensive from the point of view of manufacturing and assembly, but furthermore presents the danger, for instance when the dirt trapping filter has to be mounted again, especially during repairs, that it will be left out altogether, thereby after a certain time, leading to the failure of the check valve.